SUMMARY Many systems use only outdoor dry bulb temperature when determining the point when outdoor air requires more mechanical cooling than the return air maintaining comfort in the occupied space. At this point the dampers revert back to mainly return air, with outdoor providing only minimum ventilation. The change-over dry bulb temperature is often seventy degrees Fahrenheit. 70°F air at 100% RH contains 78.9% more heat energy than 70°F air at 15% RH. The system must sense relative humidity and dry bulb temperature in both the return air and the outdoor air and calculate the actual enthalpy in determining which air stream is more economical for mechanical cooling. SIEMENS’ enthalpy transmitter, part number 184-0101 and enthalpy logic module, 243-0043 are no longer available. The Barber Colman enthalpy transmitter, part number HKS8065, and enthalpy logic module, part number AK-52101, are no longer available. We did not test the performance of these components through their whole range, but spot checking them over time, during service, we believe that they functioned properly as enthalpy comparators. Honeywell made an enthalpy comparator, part number HP973A, which is an obsolete item now. We have never seen one in the field. Johnson Controls Enthalpy Logic Centre, part number N9000, is still commercially available. We do not consider it a true enthalpy comparison circuit, as the dry bulb temperature can cause the unit to switch from free cooling, while a change in the relative humidity will not affect that decision. Enthalpy is a combination of dry bulb temperature and relative humidity; therefore, both must continually have an impact on the decision to choose return air or outdoor air as the primary component of the air passing through the mechanical cooling equipment. We suggest that each circuit, whether pneumatic or DDC, claiming enthalpy comparison, be checked with simulated temperatures and relative humidity levels. The operator, using the psychrometric chart on page 8-107, can select various enthalpy values for the return air and the outdoor air commanding particular dry bulb temperatures and relative humidity values. The positioning of the mixed air dampers will demonstrate the true logic of the enthalpy system. 8-115b
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